1. Field of the Invention
In a first embodiment, the present invention relates to two-component aqueous polymer systems which are crosslinkable at ambient temperatures through the Michael reaction, methods of making acetoacetylated latexes, water-dispersible acetoacetylated acrylic resins and water-dispersible urethane polyacrylate resins, and to compositions for use of the polymer systems in water-borne coatings.
In a second embodiment, the present invention also relates to water-dispersible acrylated urethane acrylic (WDAUA) film-forming binders comprised of an acrylic copolymer having pendant urethane side chains terminated with (meth)acrylate groups for crosslinking, a method for preparing such WDAUA binders, and to compositions for use of the WDAUA binders with polyacetoacetates, polyketimines or photoinitiators in ambient temperature-curable or UV-curable water-borne coatings.
In a third embodiment, the present invention also relates to water-borne polymer compositions crosslinkable at ambient temperatures through dual cure of the Michael reaction and the epoxy reaction.
2. Discussion of the Background
Acetoacetylated polymers have recently become of interest in thermosetting coatings systems because they are crosslinkable with many other reactants at ambient temperatures. The known reagents which react readily with acetoacetyl groups include amines, aldehydes, metal ions and acrylate esters. The base-catalyzed addition reaction between the active methylene group of the acetoacetyl functionality and an activated olefin unsaturated double bond such as in an acrylate ester is known as the Michael reaction.
U.S. Pat. No. 4,408,018 describes crosslinking butyl acrylate/acetoacetoxyethyl methacrylate (AAEM) copolymers with polyacrylates such as trimethylolpropane triacrylate (TMPTA) in the presence of a strong basic catalyst at ambient temperatures.
Two-package acrylic urethane compositions based on hydroxyl functional acrylic copolymers and crosslinking agents containing isocyanate (NCO) groups are known as ambient temperature-curable compositions that can be used for high quality coatings with outstanding performance properties. However, the use of isocyanate crosslinkers often requires precautions to be taken in handling and using these materials due to their high toxicity. Health and environmental concerns have led researchers to develop NCO-free urethane compositions that can be cured at ambient temperatures but without involving the reactions of free NCO groups. Because (meth)acrylate groups react readily with active methylene groups such as contained in acetoacetates and primary amino groups at ambient temperatures or with themselves through free radical polymerization initiated by UV radiation and certain free radical initiators, (meth)acrylate functionalized polyurethanes have found applications in ambient temperature-curable NCO-free compositions for production of crosslinked urethanes polymers.
Thus, U.S. Pat. No. 4,078,015 describes ultraviolet (UV)-curable compositions containing 50 to 90 wt. % of an acrylated polyurethane prepared by reacting an organic diisocyanate, a hydroxyalkyl acrylate and a polyol.
U.S. Pat. No. 4,246,391 describes a process for making acrylated urethanes of low viscosity. The process involves first reacting a monohydroxyl functional acrylate with a polyisocyanate followed by reacting the product mixture with a polyol.
European Patent 0,542,219 A2 discloses a process for preparing (meth)acrylated polyurethanes from certain liquid hydrocarbon diols, polyfunctional isocyanates and hydroxyalkyl (meth)acrylates. The (meth)acrylated polyurethanes are useful as decorative and functional coatings, inks, adhesives, sealants and formed parts. The compositions containing the (meth)acrylated polyurethane are cured by UV radiation.
U.S. Pat. No. 5,132,367 discloses a NCO-free two-component polyurethane system comprised of an acetoacetate functionalized acrylic copolymer and a urethane crosslinking agent containing acrylate groups. The two-component NCO-free urethane compositions can be cured at ambient temperatures through the Michael reaction using a strong base as catalyst and are useful as a substitute for conventional two-component urethane compositions that are cured through NCO reactions.
The (meth)acrylated urethanes described in the above patents are generally prepared by first capping a polyol with diisocyanates and then capping remaining free NCO groups with hydroxyalkyl (meth)acrylates. The polyols used are usually hydrocarbon polyols, ether polyols or ester polyols containing 2 or 3 hydroxyl groups. The (meth)acrylated urethanes thus-obtained have low acrylol functionality usually 2 or at most 4. High molecular weight (&gt;2000) acrylated urethanes of low functionality (or high acrylate equivalent weight) are good film-forming binders but provide low crosslinking density. High crosslinking density is needed to give coating films with desirable performance properties. On the other hand, low molecular weight (&lt;1000) acrylated urethanes of low functionality (or low acrylate equivalent weight) can provide desired high crosslinking density, but are generally poor film-forming binders. In such case, other high molecular weight polymers must be incorporated in the coating compositions to assist film-forming.
Accordingly, it is desirable to have high molecular weight (meth)acrylated urethanes of high functionality that can serve as film-forming binders and provide desired high crosslinking density. It is further desirable to make such high molecular weight, high functionality acrylated urethanes water-dispersible owing to growing environmental concern and strict governmental regulations. Advantages associated with water-borne systems include reduction of volatile organic compounds (VOC), reduced fire and health hazards, and ease of clean up.
U.S. Pat. No. 5,177,152 discloses a process for production of a water-dispersible acrylic copolymer having pendant carboxylic acid groups and acrylate functional groups. The process involves first forming an acrylic backbone copolymer with pendant epoxy groups by copolymerization with glycidyl methacrylate, then reacting the epoxy groups with acrylic acid to introduce acrylol functionality and finally reacting the hydroxyl groups (produced from the epoxy-carboxylic acid reaction and/or from the copolymerized hydroxyethyl methacrylate) with a dibasic acid anhydride to introduce carboxyl groups for water dispersibility. The same procedure of preparing water dispersible ethylenically unsaturated acrylic copolymers was also reported in Progress in Organic Coatings, vol. 17, 27-39 (1988), by A. Noomen.
The water-dispersible acrylated acrylic resins so obtained are high molecular weight polymers and can have high acrylate functionality. However, they are not urethane-type resins and will not give the advantages of crosslinked polyurethanes. Besides, these resins have relative dark color and low solid content (about 50 wt. %). The low solid or high organic solvent content limits the use of these resins, as obtained, for low VOC coatings. The water-dispersible acrylated acrylic resins described in U.S. Pat. No. 5,177,152 are used with polyamines or polyketimines as crosslinkers to prepare two-component ambient cured coatings. However, the two-component water-borne coatings using polyamines or polyketimines crosslinkers have a short pot-life (less than 2 hours).
Thus, there remains a need for water-dispersible acrylated urethanes of high molecular weight and high functionality which can overcome the above-mentioned shortcomings and provide the outstanding performance of urethane materials.
European Patent 0,326,723 B1 teaches the use of tertiary amines with epoxides as potential catalysts for the Michael reaction cure in solvent-borne systems. The tertiary amines used, typically, triethylene diamine (TEDA), are relatively weak bases and cannot be used alone as catalysts for the Michael reaction. It is believed that in the presence of an activated methylene component, the reaction between amine and epoxide forms, in situ, a stronger base of quaternary ammonium which then functions as the Michael reaction catalyst to activate the methylene component for reaction with the active alkene component. The tertiary amine-epoxide catalyst system improves the drawback of short pot-life for solvent-borne Michael reaction cure systems using strong base catalyst.
European Patent 0,326,723 B1, however, does not cover the use of epoxides in strong base-catalyzed Michael reaction cure systems to achieve synergistic effects of Michael reaction cure and epoxy cure on coatings peformance properties. The epoxides in this patent are mainly used as co-activators with tertiary amines to generate a strong base in-situ for the Michael reaction cure. No mention is made that the epoxides function as a third component and participate in forming a crosslinked network with the active methylene component and active alkene component to contribute to coatings performance properties. In fact, the use of low amounts of epoxides is encouraged for better coatings properties. Epoxides are generally used in equivalent molar amount with tertiary amines. High levels of epoxide requires high levels of the tertiary amine. The presence of excessive amine gives a shorter pot-life and is detrimental to acid resistance and weathering resistance. Use of excessive equivalents of epoxide to amine in these systems resulted in poorer coatings performance properties such as hardness and solvent resistance.
Although European Patent 0,326,723 B1 mentions use of some acetoacetylated copolymers with a small amount of carboxyl content (0.5% methacrylic acid), the use of polymers with higher carboxyl content as can be used in water-borne coating compositions is not disclosed.
The patents mentioned above deal only with solvent-borne systems. Solvent-borne coatings cured through the Michael reaction suffer from the drawback of having a short pot-life. In addition, the growing environmental concern and strict government regulations have made it desirable to develop corresponding water-borne systems. The advantages associated with water-borne systems are obvious: low volatile organic compounds (VOC), less fire and health hazards, and ease to clean up.